The present invention relates to a system for controlling an electromagnetic clutch for an internal combustion engine mounted on a vehicle, and more particularly to a system for controlling clutch current to a predetermined value in spite of the variation of the idling speed during idling operation of the engine.
An electromagnetic powder clutch as a type of electromagnetic clutch is known. The electromagnetic powder clutch comprises an annular drive member secured to the crankshaft of an engine, a driven member secured to the input shaft of a transmission and provided adjacent to the drive member spaced therefrom by a small gap, a magnetizing coil provided in one of the members, and magnetic powder provided in a chamber in the clutch. Change speed gears in the transmission are changed by operating a shift lever. The shift lever is provided with a switch for the circuit of the magnetizing coil, when the shift lever is gripped by operator's hand, the switch is opened to cut off the clutch current. Accordingly, the gear changing operation of the transmission may be performed. When the shift lever is shifted to a gear engaging position, and released from the hand, of the operator the switch is closed so that electric current flows through the magnetizing coil to magnetize the member provided with the magnetizing coil. As the accelerator pedal is depressed, the current applied to the coil increases. The magnetic powder is aggregated in the gap between the drive member and the driven member, so that the driven member is coupled to the drive member. The clutch current passing through the magnetizing coil is progressively increased according to the degree of depression of the accelerator pedal, while the clutch slips between the drive member and driven member and gradually engages to full engagement when the clutch current reaches to the rated current. Thus, the vehicle may be started by depressing the accelerator pedal without operating a clutch pedal.
In order to control driving of the vehicle, it is necessary to detect the speed of the vehicle and to control the clutch current according to the speed. In a system to meet such a requirement, a speed sensor is provided to generate a signal when the vehicle speed exceeds a predetermined value. The system is designed such that the clutch slips for providing partial engagement of the clutch during low speed below the predetermined speed and the clutch fully engages when the vehicle speed reaches the predetermined speed. When the vehicle speed decreases to a predetermined low value, the clutch is disengaged so as to prevent the engine from stopping. Generally, the speed sensor is provided in the speedometer operated through a speedometer cable. In a system employing such a speed sensor, there is the disadvantage that when the vehicle is decelerated to a speed below the predetermined speed, the braking action of the engine is not effective because of the disengagement of the clutch. Further, if the speedometer cable is broken, the speed sensor does not work. Therefore, if the speed sensor does not work in a deceleration condition, the circuit of the magnetizing coil of the clutch is interrupted so that the clutch disengages. As a result, the braking action of the engine is not effective.
In order to eliminate such disadvantages, a control system employing a speed detecting means which detects the engine speed from ignition pulses of the engine has been proposed. In accordance with this control system, pulsating current passes through the magnetizing coil of the clutch and the current is controlled in proportion to the ignition pulses for providing the partial engagement of the clutch.
FIG. 11 shows the relationship between the clutch torque and the engine speed in the conventional system. In a normal engine operation, the clutch torque varies as shown by a curve C. When the accelerator pedal is depressed, the vehicle starts with a torque A at the corresponding speed in the idling operation. If the choke valve of the engine is closed or an air conditioning device of the vehicle is operated, the engine speed is increased and the clutch torque for starting the vehicle is increased to B on the curve D at the corresponding idling speed. Accordingly, the vehicle starts abruptly by the high clutch torque and cannot drive at a very low speed.